Butadiene is normally produced during the pyrolysis of petroleum fractions. For example, producing ethylene and propylene by pyrolysis of petroleum gas, naphtha, or gas oil in a steam cracker also yields secondary products that include significant amounts of a C4 hydrocarbon fraction that is rich in 1,3 butadiene and butenes and has minor amounts of acetylenes. Conventional distillation however cannot separate 1,3 butadiene with the required purity from this C4 hydrocarbon fraction due to the closeness of boiling points and, thus, small differences in the relative volatilities of the components.
Extractive distillation (ED) has long been the only method of choice for recovering high purity 1,3 butadiene from the close-boiling C4 hydrocarbon mixture. This method entails adding a selective solvent (also referred to as an extractant), which has a boiling point that is significantly higher than that of the C4 mixture, to the distillation column to increase the differences in the relative volatilities of the components to be separated from one another. A single extractive distillation column (EDC) however yields only crude butadiene that contains acetylenes, which are deleterious to the polymerization of 1,3 butadiene, and other troublesome impurities.
There are two primary approaches for subsequently removing the major impurities from the crude butadiene: (1) selective hydrogenation of the acetylenes in a separate reactor or column and (2) selective removal of the acetylenes in a second EDC followed by additional distillations, where necessary. Both strategies for removing acetylene impurities require crude butadiene production with an initial EDC. Examples of the selective hydrogenation approach are disclosed U.S. Pat. No. 3,541,178 to Nettesheim, U.S. Pat. No. 3,842,137 to Dickenson, U.S. Pat. No. 3,898,298 to Desiderio et al., U.S. Pat. No. 4,277,313 to Mehra, U.S. Pat. No. 4,469,907 to Araki et al., U.S. Pat. No. 4,704,492 to Nemet-Mavrodin, U.S. Pat. No. 5,414,170 to McCue et al., U.S. Pat. No. 6,040,489 to Imai and U.S. Pat. No. 7,393,992 to Hill et al. A technique embodying the second approach is described in U.S. Pat. No. 4,128,457 to Barba et al. whereby a second EDC using aqueous acetonitrile as the selective solvent and downstream distillation columns are employed to remove the acetylenes. The second EDC represents a major capital investment and another energy intensive operation.
Instead of using a second EDC for acetylenes removal, U.S. Pat. No. 4,038,156 to Knott et al. describes a process configuration that features flashing a rich solvent containing 1,3 butadiene, acetylenes and minor amounts of C5 hydrocarbons from the bottom of an EDC into a flash drum and feeding a liquid stream from the flash drum to a first stripping column where acetylenes are removed from a side-cut, lean solvent is removed from the bottom for recycling to the EDC, and vapor containing 1,3 butadiene, traces of C5 and the solvent is removed from the top. A mixed vapor stream from the top of the first stripping column and from the flash drum is compressed and most of the compressed vapor is fed to a second stripping column to recover purified 1,3 butadiene. The overall process is also capital and energy intensive since it requires an EDC, two distillation columns (one for water separation), two water wash columns, and two stripping columns.
Solvent capability limitations restrict the degree to which the number and size of separation trays in an EDC can be reduced and still recover high purity 1,3 butadiene from C4 fractions. The art is in need of minimizing utilities or energy consumption while enhancing the throughput of existing EDCs without major equipment revamping and capital expenditures.